Christ. Since when do TV shows last precisely as long as the premise logically allows?
It's an NBC show. It'll go 'til it gets cancelled and then it'll stop, and that will be decided by the network based on ratings not when they "graduate" or when they're out of stories or whatever.
Also, if you NEEEED to justify it, it'd community college
. People can go to community college as long as they please, whether they're getting a degree or not. Doesn't matter. That's the point.